Breast cancer is one of the leading causes of cancer death among women. It’s caught early about 95 percent of the time, and a relatively simple operation can remove the tumor. Chemotherapy can follow surgery to try to kill cancer cells that might have gotten away.

But some women who seem to be cured experience a recurrence, with metastatic cancer cells re-establishing tumors at distant sites, including the liver and brain.

“What we know unfortunately is chemotherapy doesn’t work ... in everybody," said Nicholas Turner, an oncologist at the Institute of Cancer Research in London. "And what we need are tests to help work out whether there’s cancer left after chemotherapy to help us direct treatment.”

In an article published in Science Translational Medicine, Turner and his colleagues describe being able to detect DNA from tumor cells circulating in the blood. The technique uses highly sensitive genetic sequencing to track tumor-specific mutations from cells that split off from the original cancer.

Investigators took regular blood samples for two years from 55 women with early-stage breast cancer who had been treated with surgery and chemotherapy and were considered cancer-free.

Eventually, 15 patients relapsed. But researchers had detected the cancer DNA in a dozen of the women an average of eight months before the tumors could be seen with mammograms or other conventional imaging.

For now, Turner said, not much can be done to help patients whose tumors have spread, "so what we need to be able to try to do is identify them much earlier in the point where there are relatively few cancer cells, where treatments stand a chance of potentially curing it.”

And because the test focuses on mutations in the tumor cells, researchers say that information could help doctors select the most effective drugs for targeting the cancer.